The heart does not have the de novo system of fatty acid synthesis and actively takes up fatty acids. The mitochondrial chain elongation is the only fatty acid synthetic system present in the heart. Our studies have shown that the de novo system as well as the mitcochondrial chain elongation of endogenous fatty acids is completly inhibited by the antilipagenic antibiotic, cerulenin. If cerulenin inhibits mitochondrial chain elongation in the cltured heart cells, then it is possible to culture heart cells with defined membrane fatty acid composition by growing these cells in the presence fo selected fatty acids and cerulenin. It is proposed to investigate the above hypothesis and to study the mitochondrial chain elongation in the heart. Our investigation on the hormonal regulation of fatty acid synthetase and stearyl CoA desaturase in the newly hatched chicks will be correlated to similar studies in cultured liver explants and cells.